A maize crop affected by long fallow disorder. The plants at left are growing in a patch where weeds were uncontrolled for a period in the previous fallow.
Soils naturally contain beneficial fungi that help a crop access nutrients like phosphorus and zinc. The combination of the fungus and crop root is known as arbuscular mycorrhiza (AM). Many different species of fungi can have this association with the roots of crops. Many that are associated with crops also form structures called vesicles in the roots and are called vesicular arbuscular mycorrhizae (VAM).
The severe reduction or lack of VAM shows up as long fallow disorder - the failure of crops to thrive despite adequate moisture. Ongoing drought in the 1990s and beyond has highlighted long fallow disorder where VAM has died out through lack of host plant roots during long fallow periods. As cropping programs restart after dry years, an unexpected yield drop is likely due to reduce VAM levels, making it difficult for the crop to access nutrients.
Long fallow disorder is usually typified by poor crop growth. Plants seem to remain in their seedling stages for weeks and development is very slow. Sorghum and maize is generally pale, with some purpling and yellow/brown areas on older leaves. Sunflowers are pale and unthrifty. Mungbeans remain very short and there is often an upward cupping of the youngest leaves. Additionally, because mungbeans are such a quick crop, they are less likely to recover from LFD.
Sorghum is not as sensitive to LFD as maize, sunflower or mungbeans. LFD tends to significantly slow down the rate of development of all crops, but sorghum seems to be capable of reasonable recovery.
If LFD is likely, using starter fertilisers containing phosphorus and zinc is the best option, though does not always correct the symptoms. Starter fertiliser must go down the same tube as the seed. Consult an experienced agronomist to establish appropriate fertiliser rates for your situation.
These mungbean plants (below right) are more than weeks old. The plant at left was taken from a patch of couch grass in the paddock. The plant at right was located nearby and is suffering from LFD.
These mungbean plants are more than four weeks old. The plant at left was taken from a patch of couch grass in the paddock The plant at right was located nearby and is suffering from LFD.
Benefits of good VAM levels are:
- improved uptake of phosphorus and zinc
- improved crop growth
- improved nitrogen fixation
- greater drought tolerance
- improved soil structure
- greater disease tolerance.
In general, the benefits of VAM are greater at lower soil phosphorus levels because VAM increases a plant's ability to access this nutrient. Crops with higher dependency benefit more from VAM.
|Potential yield loss
without mycorrhiza (%)
|Very high||Greater than 90||Linseed|
|High||60-80||Sunflower, mungbean, pigeon
pea, maize, chickpea
|Medium||40-60||Sudan, sorghum, soybean|
|Low||10-30||Wheat, barley, triticale|
|Very low||0-10||Panicum, canary|
Pulses (including mungbean, peanuts, lablab, pigeon pea, fababean, soybean, chickpea and navybean) are usually highly to very highly VAM-dependent. Pulse crops can perform a valuable role in crop rotations by leaving high levels of VAM spores in the soil to colonise the next crop. Oilseed crops, such as linseed, sunflower and safflower, are very highly dependent.
Cotton depends very highly on VAM for growth. At a soil phosphorus level of 8 mg/kg, cotton has a very high mycorrhizal dependency of 89 per cent. At 18 mg/kg soil phosphorus, the dependency is 54 per cent.
VAM cannot manufacture phosphorus or zinc. It can access the nutrients in the soil better than roots alone. P and Zn fertilisers are used more efficiently with VAM present. In soil with a lower phosphorus background, phosphorus fertiliser may need to be added, even with good VAM levels.
Optimising the benefits from VAM depends on knowing paddock nutrient status (the available P and Zn levels), VAM dependency of proposed crop and paddock cropping history (including length of fallow periods and previous). From this, you can estimate the VAM inoculum level in the soil.
If you expect low VAM, grow crops with low or very low VAM dependency. These crops will not suffer much yield loss but will still increase the VAM inoculum for following crops. If you wish to grow a crop of high dependency for reasons such as price, apply high rates of phosphorus and zinc fertilisers. If you expect high VAM, grow the most profitable crop. If possible, grow a crop that produces a lot of VAM spores so that highly dependent crops are future options.
Opportunity crop whenever moisture is available to maintain mycorrhiza. A green manure crop will enhance mycorrhiza. VAM inoculum builds up during cropping but not all crops produce equal quantities of inoculum. For example, grain crops are very valuable for use in a rotation with cotton.
These sorghum plants were growing within metres of each other. The plant at left is from a weedy patch, while the plant at right is affected with LFD.
Farm practices can damage VAM populations
In Queensland, fallows are usually maintained weed free to conserve soil moisture. Fallows over 12 months can result in long fallow disorder. Severity varies to a crop's mycorrhizal dependency and the fertility status of the soil. Weedy fallows may prevent VAM decline but the weeds use valuable water and nitrate reserves. Some weeds, such as those from the cabbage family (e.g. turnip weed, wild turnip, wild radish) and the amaranths (e.g. red shank, native amaranth) are non-mycorrhizal.
VAM inoculum builds up during cropping but not all crops produce equal quantities of inoculum. This affects the growth of the next crop. Some crops, such as canola and lupins, are not hosts for VAM fungi so they behave like a weed-free fallow.
Tillage, soil disturbance and land levelling
Severe soil disturbance can damage VAM, reducing VAM inoculum and colonisation in subsequent crops. A single aggressive tillage with a rotary hoe, for example, can severely affect growth of the following crop. VAM populations are greatest in the topsoil, so removal through erosion, land clearing or construction of contour banks can result in loss of VAM inoculum and poor colonisation of the next crop.
Fire and heating
Burning windrows of stubble or timber can heat soil to temperatures that kill VAM fungi.
Excessive soil waterlogging results in poor root colonisation with VAM due to a lack of oxygen.
Broad spectrum fumigants and some fungicides are very toxic to VAM under ideal soil moisture and temperature conditions. Recommended rates of insecticides and nematicides generally do not inhibit VAM fungi.